22.1.4.1 Array parameters are pointers

Declaring a function parameter variable as an array really gives it a pointer type. C does this because an expression with array type, if used as an argument in a function call, is converted automatically to a pointer (to the zeroth element of the array). If you declare the corresponding parameter as an “array”, it will work correctly with the pointer value that really gets passed.

This relates to the fact that C does not check array bounds in access to elements of the array (see Accessing Array Elements).

For example, in this function,

void
clobber4 (int array[20])
{
  array[4] = 0;
}

the parameter array’s real type is int *; the specified length, 20, has no effect on the program. You can leave out the length and write this:

void
clobber4 (int array[])
{
  array[4] = 0;
}

or write the parameter declaration explicitly as a pointer:

void
clobber4 (int *array)
{
  array[4] = 0;
}

They are all equivalent.